Method of transmitting traffic in a communications network and communications network apparatus

ABSTRACT

A method  10  of transmitting traffic in a communications network. The method comprises receiving client signal having a client bit rate and comprising traffic to be transmitted  12  and monitoring an indicator indicative of said client bit rate  12 . The method further comprises setting a transmission bit rate at which to transmit the traffic  16 . The transmission bit rate is set in dependence on said client bit rate. The method further comprises generating and transmitting a control signal arranged to cause a transmission apparatus to transmit the traffic at the transmission bit rate  18.

This application is a continuation of International Application No.PCT/EP2010/062858 filed 2 Sep. 2010, which designated the U.S., theentire contents of which is hereby incorporated by reference.

TECHNICAL FIELD

The invention relates to a method of transmitting traffic in acommunications network. The invention further relates to acommunications network transmission element. The invention furtherrelates to a communications network element. The invention furtherrelates to a communications network. The invention further relates to amethod of framing traffic for transmission in a communications network.The invention further relates to a method of controlling transmission oftraffic in a communications network.

BACKGROUND

Improving the power efficiency of electrical products is receiving anincreasing amount of attention both for consumer goods and professionalgoods. Telecommunications operators are beginning to specify minimumpower efficiency for the products they buy and Standards bodies fortelecommunications are introducing features to improve power efficiency.As yet, telecommunications transmission products have not beeneffectively addressed and many of them consume the same amount of powerirrespective of the amount of traffic they are carrying. For example, inpacket networks transponders make up the bulk of the power consumptionrequirements and use the same power regardless of whether the opticalchannel payload unit (OPU) content comprises empty or filled packets.Burst mode transmission has been proposed to reduce power consumptionwhen little traffic is to be carried by turning off the transmitterbetween bursts. However this requires additional overhead in order toprovide clock and burst synchronisation. In addition, burst modetransmission is incompatible with most installed wavelength divisionmultiplexed (WDM) communications network infrastructure, that has beendesigned for continuous transmission. For example, Erbium doped fibreamplifiers and power monitors used in the optical transport network aregenerally unsuitable to carry burst mode transmission.

SUMMARY

It is an object to provide an improved method of transmitting traffic ina communications network. It is a further object to provide an improvedcommunications network transmission element. It is a further object toprovide an improved communications network element. It is a furtherobject to provide an improved communications network. It is a furtherobject to provide an improved method of framing traffic for transmissionin a communications network. It is a further object to provide animproved method of controlling transmission of traffic in acommunications network.

A first aspect of the invention provides a method of transmittingtraffic in a communications network. The method comprises receiving aclient signal having a client bit rate and comprising traffic to betransmitted and monitoring an indicator indicative of said client bitrate. The method further comprises setting a transmission bit rate atwhich to transmit the traffic. The transmission bit rate is set independence on said client bit rate. The method further comprisesgenerating and transmitting a control signal arranged to cause atransmission apparatus to transmit the traffic at the transmission bitrate.

The method may enable the transmission bit rate to be reduced inresponse to receiving a client signal having a client bit rate which isless than a maximum transmission bit rate at which traffic can betransmitted across the network. Reducing the transmission bit rate mayreduce power consumption during transmission of traffic. The method mayenable power consumption to be varied to match the received trafficload. The method may enable traffic transmission to continuecontinuously but at a lower transmission bit rate than the maximum forthe network or the capability of the transmission apparatus and may thusenable the method to be used in connection with currently installedwavelength division multiplexed network infrastructure.

In an embodiment, the transmission bit rate is set by determining adivide ratio for a master bit rate in dependence on said client bitrate. The control signal comprises a clock signal generated by receivinga master clock signal at the master bit rate and dividing the masterclock signal by the divide ratio. This may provide a simple mechanismfor setting the transmission bit rate and generating the control signal.The transmission bit rate is varied by varying the divide ratio whilethe master clock signal has a constant master bit rate. This may easeclock recovery at a receiver which can run at a constant bit rate lockedto an incoming signal.

In an embodiment, the bit rate is set by selecting a divide ratio from aset of divide ratios in dependence on said quantity of traffic. This mayfurther simplify operation of the method as it is only necessary toselect an appropriate one of a set of divide ratios rather thandetermining a divide ratio. In an embodiment, an intermediate bit rateis set by periodically switching between each of a plurality of divideratios selected from the set of divide ratios. The intermediate bit ratecomprises an average of the bit rates at the said divide ratios.

In an embodiment, the method comprises receiving a first client signalhaving a first client bit rate and comprising first traffic to betransmitted and subsequently receiving a second client signal having asecond client bit rate and comprising second traffic to be transmitted.A first transmission bit rate is set by monitoring an indicatorindicative of the first client bit rate and a second transmission bitrate is set by monitoring an indicator indicative of the second clientbit rate. The method further comprises generating and transmitting afirst control signal and a second control signal. The first controlsignal is arranged to cause a framer to frame the first traffic into oneor more first frames at the first transmission bit rate. The secondcontrol signal arranged to cause the framer to frame the second trafficinto one or more subsequent frames at the second transmission bit rate.Where the second transmission bit rate is different to the firsttransmission bit rate, the second control signal is further arranged tocause the framer to implement the change from the first transmission bitrate to the second transmission bit rate at a frame boundary. Changingthe bit rate only at a frame boundary may ensure that a clean changeover is achieved from the first transmission bit rate to the secondtransmission bit rate and may prevent spillage of traffic from a firstframe into a subsequent frame.

In an embodiment, the first control signal is arranged to cause theframer to frame a part of the first traffic in a payload of a said firstframe. The first control signal is further arranged to cause the framerto provide information indicative of the second transmission bit rate inan overhead of said first frame. This may enable a receiver receivingthe traffic to track changes in the transmission bit rate and to prepareto receive traffic at a different transmission bit rate.

In an embodiment, the frame comprises an optical transport networkframe. In an embodiment, the optical transport network frame comprisesan optical transport network frame as defined in ITU-T RecommendationG.709. In an embodiment, the control signal is arranged to cause theframer to provide information indicative of the second transmission bitrate in one of: byte 15 of one of rows 1 and 2 of the overhead of saidfirst frame; one of bytes 13 to 15 of row 3 of the overhead of saidfirst frame; and one of bytes 9 to 14 and 16 of row 4 of the overhead ofsaid first frame. The information may therefore be provided in anyavailable bytes of the overhead which are not being used in connectionwith transmission of the traffic, including byte 16 in rows 1 to 4 whichare usually allocated for justification control but which are notrequired in this method since justification control is only requiredwhen transporting constant bit rate traffic.

In an embodiment, the control signal is arranged to cause the framer toprovide said information indicative of the second transmission bit ratein a first plurality of said bytes. The first plurality of bytes islocated in a second plurality of rows of the overhead. This may provideredundancy against transmission errors.

In an embodiment, the information indicative of the second transmissionbit rate comprises a plurality of bits coded to indicate that the secondbit rate comprises one of: a master bit rate; one half of the master bitrate; one third of the master bit rate; and one quarter of the masterbit rate.

In an embodiment, the information indicative of the second transmissionbit rate comprises three bits coded as one of: 001 to indicate themaster bit rate; 010 to indicate one half of the master bit rate; 100 toindicate one third of the master bit rate; and 110 to indicate onequarter of the master bit rate.

In an embodiment, the method further comprises receiving transmittedtraffic at receiver apparatus and determining whether a value for thetransmission bit rate is available. If a value for the transmission bitrate is not available, the method further comprises causing a furthertransmission apparatus to generate and transmit a recovery signal. Therecovery signal is arranged to cause the transmission apparatus totransmit traffic at a default transmission bit rate. Backwardinformation may therefore be sent from the receiver apparatus to thetransmission apparatus to inform the transmission apparatus that thereceiver apparatus has lost the bit rate. Transmitting traffic at adefault transmission bit rate may ease regaining of transmission bitrate and frame synchronisation at the receiver apparatus. In anembodiment, the recovery signal is arranged to cause the remotetransmission apparatus to re-transmit the traffic on a transport signalat the default transmission bit rate. The risk of losing traffic forwhich the transmission bit rate was not available at the receiverapparatus may therefore be reduced.

In an embodiment, the recovery signal is transmitted at a pre-determinedrecovery signal transmission bit rate. The method comprises framing therecovery signal in a payload of a frame and providing a recovery signalidentifier in an overhead of the frame. In an embodiment, the recoverysignal identifier comprises two bits coded as 10.

In an embodiment, the receiver apparatus comprises a receiver softdecision and equaliser and the method further comprises providing thetransmission bit rate to the receiver soft decision and equaliser. Thismay make it easier to predict threshold levels for the soft decision andequaliser.

In an embodiment, the method comprises receiving the or each said clientsignal in an input traffic queue buffer and monitoring the rate at whichthe queue buffer is filled. In an embodiment, the rate at which thequeue buffer is filled is monitored by monitoring when a fill level ofthe queue buffer reaches a threshold fill level. In an embodiment, theinput traffic queue buffer comprises a first-in-first-out buffer. Usingan input traffic queue buffer may reduce latency variation because whenthe queue buffer is filled traffic is transmitted at a highertransmission bit rate.

In an embodiment, the indicator comprises a client bit rate signalindicative of the client bit rate. In an embodiment, the client bit ratesignal is received from a network management system. In an embodiment,the communications network comprises an optical transport network. Thetransmission bit rate is determined by an optical channel transport unitclock rate. In an embodiment, the communications network comprises asynchronous digital hierarchy network. The transmission bit rate isdetermined by a synchronous transport module of the network.

In an embodiment, the communications network is an optical packetnetwork.

In an embodiment, the method further comprises de-framing the receivedtransmitted traffic and holding the de-framed traffic in a queue bufferuntil a complete packet of traffic has been received, and thendelivering the de-framed traffic to an output client signal interface.This may prevent packet under-run when received transmitted traffic hasa lower transmission bit rate than the output client signal bit rate

In an embodiment, the method further comprises obtaining a temperatureof the transmission apparatus and the divide ratio is further set independence on the temperature. Reducing the transmission bit rate independence on the ambient temperature may reduce the power required forcooling the transmission apparatus. The method may therefore be used tothrottle the transmission bit rate if the ambient temperature risesabove a threshold value. This may be implemented when the client bitrate is lower than the maximum transmission bit rate capability of thetransmission apparatus.

A second aspect of the invention provides a communications networktransmission element comprising an input, client signal monitoringapparatus, transmission bit rate control apparatus and transmissionapparatus. The input is arranged to receive a client signal having aclient bit rate and comprising traffic to be transmitted. The clientsignal monitoring apparatus is arranged to monitor an indicatorindicative of said client bit rate. The transmission bit rate controlapparatus is arranged to set a transmission bit rate at which totransmit the received traffic. The transmission bit rate is set independence on said client bit rate. The transmission bit rate controlapparatus is further arranged to generate and transmit a control signalarranged to cause a transmission apparatus to transmit the traffic atthe transmission bit rate. The transmission apparatus is arranged toreceive the control signal and to generate and transmit a transportsignal carrying the traffic.

The transmission element may be able to reduce the transmission bit ratein response to receiving a client signal having a client bit rate whichis less than a maximum transmission bit rate capability of thetransmission element. Reducing the transmission bit rate may reducepower consumption in the transmission element, particularly in thetransmission apparatus. The power consumption of the transmissionelement may therefore be varied to match the received traffic load. Thetransmission element may enable traffic transmission to continuecontinuously but at a lower transmission bit rate than its maximumcapacity and may thus be used within currently installed wavelengthdivision multiplexed network infrastructure.

In an embodiment, the transmission bit rate control apparatus comprisesa controller, a clock and a divider. The controller is arranged todetermine a divide ratio in dependence on said client bit rate. Theclock is arranged to generate a master clock signal having a master bitrate. The divider is arranged to receive the divide ratio and to dividethe master clock signal at the divide ratio to form the control signal.The transmission bit rate control apparatus may therefore operate with asimple mechanism for setting the transmission bit rate and generatingthe control signal. The transmission bit rate is varied by varying thedivide ratio while the master clock signal has a constant master bitrate. This may ease clock recovery at a receiver which can run at aconstant bit rate locked to an incoming signal.

In an embodiment, the divide ratio is selected from a set of divideratios in dependence on said quantity of traffic. This may furthersimplify operation of the transmission bit rate control apparatus as itmust only select an appropriate one of a set of divide ratios ratherthan determining a divide ratio. In an embodiment, the controller isarranged to set the bit rate at an intermediate bit rate by periodicallyswitching between each of a plurality of divide ratios selected from theset of divide ratios. The intermediate bit rate comprises an average ofthe bit rates at the said divide ratios.

In an embodiment, the input is arranged to receive a first client signalhaving a first client bit rate and comprising first traffic to betransmitted and to subsequently receive a second client signal having asecond client bit rate and comprising second traffic to be transmitted.The transmission apparatus comprises a framer. The transmission bit ratecontrol apparatus is arranged to set a first transmission bit rate independence on the first client bit rate at which to transmit the firsttraffic. The transmission bit rate control apparatus is arranged to seta second transmission bit rate in dependence on the second client bitrate at which to transmit the second traffic. The transmission bit ratecontrol apparatus is further arranged to generate and transmit a firstcontrol signal arranged to cause the framer to frame the first trafficinto one or more first frames at the first transmission bit rate. Thetransmission bit rate control apparatus is further arranged to generateand transmit a second control signal arranged to cause the framer toframe the second traffic into one or more subsequent frames at thesecond transmission bit rate. Where the second transmission bit rate isdifferent to the first transmission bit rate, the second control signalis further arranged to cause the framer to implement the change from thefirst transmission bit rate to the second transmission bit rate at aframe boundary. Causing the bit rate to change only at a frame boundarymay ensure that a clean change over is achieved from the firsttransmission bit rate to the second transmission bit rate and mayprevent spillage of traffic from a first frame into a subsequent frame.

In an embodiment, the first control signal is arranged to cause theframer to frame a part of the first traffic in a payload of a said firstframe. The first control signal is further arranged to cause the framerto provide information indicative of the second transmission bit rate inan overhead of the first frame. This may enable a receiver receiving thetraffic to track changes in the transmission bit rate and to prepare toreceive traffic at a different transmission bit rate.

In an embodiment, the frame is an optical transport network frame. In anembodiment, the optical transport network frame comprises an opticaltransport network frame as defined in ITU-T Recommendation G.709. In anembodiment, the control signal is arranged to cause the framer toprovide information indicative of the second transmission bit rate inone of: byte 15 of one of rows 1 and 2 of the overhead of said firstframe; one of bytes 13 to 15 of row 3 of the overhead of said firstframe; and one of bytes 9 to 14 and 16 of row 4 of the overhead of saidfirst frame. The information may therefore be provided in any availablebytes of the overhead which are not being used in connection withtransmission of the traffic, including byte 16 in rows 1 to 4 which areusually allocated for justification control but which are not requiredin this method since justification control is only required whentransporting constant bit rate traffic.

In an embodiment, the control signal is arranged to cause the framer toprovide said information indicative of the second transmission bit ratein a first plurality of said bytes. The first plurality of bytes arelocated in a second plurality of rows of the overhead. This may provideredundancy against transmission errors.

In an embodiment, the information indicative of the second transmissionbit rate comprises a plurality of bits coded to indicate that the secondbit rate comprises one of: a master bit rate; one half of the master bitrate; one third of the master bit rate; and one quarter of the masterbit rate. In an embodiment, the information indicative of the second bitrate comprises three bits coded as one of: 001 to indicate the masterbit rate; 010 to indicate one half of the master bit rate; 100 toindicate one third of the master bit rate; and 110 to indicate onequarter of the master bit rate.

In an embodiment, the client signal monitoring apparatus is arranged tomonitor an indicator indicative of the client bit rate. In anembodiment, the client signal monitoring apparatus comprises an inputtraffic queue buffer. The client signal monitoring apparatus is arrangedto monitor the rate at which the queue buffer is filled. In anembodiment, client signal monitoring apparatus is arranged to monitorwhen a fill level of the queue buffer reaches a threshold fill level. Inan embodiment, the input traffic queue buffer comprises afirst-in-first-out buffer. Providing an input traffic queue buffer mayreduce latency variation within the network element because when thequeue buffer is filled traffic is transmitted at a higher transmissionbit rate.

In an embodiment, the client signal monitoring apparatus is arranged tomonitor a client bit rate signal indicative of the client bit rate. Inan embodiment, the client bit rate signal is provided by a networkmanagement system.

In an embodiment, the communications network transmission elementfurther comprises a receiver arranged to receive a recovery signal. Therecovery signal is arranged to cause the transmission apparatus totransmit a transport signal carrying traffic at a default transmissionbit rate. Backward information may therefore be received from a remotetransmission apparatus, for example at a further transmission element,to inform the transmission element that receiver apparatus at the remotetransmission element has lost the transmitted bit rate. Transmittingtraffic at a default transmission bit rate may ease regaining oftransmission bit rate and frame synchronisation at the remotetransmission element. In an embodiment, the recovery signal is arrangedto cause the transmission apparatus to re-transmit the traffic on atransport signal at the default transmission bit rate. The risk oflosing traffic for which the transmission bit rate was not available atthe remote transmission element may therefore be reduced.

In an embodiment, the communications network comprises an opticaltransport network. The transmission bit rate is determined by an opticalchannel transport unit clock rate. In an embodiment, the communicationsnetwork comprises a synchronous digital hierarchy network. Thesynchronous digital hierarchy network may be an SDH network as definedby ITU-T Recommendation G.707 and other ITU-T recommendations or a SONETnetwork as defined by American National Standards Institute (ANSI)standards. The transmission bit rate is determined by a synchronoustransport module of the network.

In an embodiment, the communications network transmission elementcomprises an optical packet communications network transmission element.

In an embodiment, the communications network transmission elementfurther comprises a thermometer arranged to measure a temperature of thetransmission apparatus and the transmission bit rate control apparatusis further arranged to set the divide ratio in dependence on thetemperature. Reducing the transmission bit rate in dependence on theambient temperature may reduce the power required for cooling thetransmission apparatus. The transmission bit rate may therefore bethrottled to control the cooling apparatus power requirements if theambient temperature rises above a threshold value. The transmission bitrate control apparatus may implement this when the client bit rate islower than the maximum transmission bit rate capability of thetransmission apparatus. A third aspect of the invention provides acommunications network element comprising an input; transport signalinterrogation apparatus and local transmission apparatus. The input isarranged to receive a transport signal carrying traffic and having atransmission bit rate. The transport signal interrogation apparatus isarranged to determine whether a value for the transmission bit rate isavailable. The transport signal interrogation apparatus is furtherarranged to, if a value for the transmission bit rate is not available,generate a recovery signal arranged to cause a remote transmissionapparatus to transmit a transport signal carrying traffic at a defaulttransmission bit rate. The local transmission apparatus is arranged totransmit the recovery signal.

The network element may therefore provide backward information to aremote transmission apparatus, for example at a further network element,to inform the transmission apparatus that the network element has lostthe bit rate. Causing the remote transmission apparatus to transmittraffic at a default transmission bit rate may ease regaining oftransmission bit rate and frame synchronisation at the network element.

In an embodiment, the recovery signal is arranged to cause the remotetransmission apparatus to re-transmit the traffic on a transport signalat the default transmission bit rate. The risk of losing traffic forwhich the transmission hit rate was not available at the network elementmay therefore be reduced.

In an embodiment, the local transmission apparatus is arranged totransmit the recovery signal is at a pre-determined recovery signaltransmission bit rate. The local transmission apparatus is arranged toframe the recovery signal in a payload of a frame and to provide arecovery signal identifier in an overhead of the frame. In anembodiment, the recovery signal identifier comprises two bits coded as10.

In an embodiment, the receiver apparatus comprises a receiver softdecision and equaliser and the transport signal monitoring apparatus isfurther arranged to provide the transmission bit rate to the receiversoft decision and equaliser. This may make it easier to predictthreshold levels for the soft decision and equaliser.

In an embodiment, the communications network element further comprises ade-framer and a queue buffer. The de-framer is arranged to receive thetransport signal and to de-frame the traffic. The queue buffer isarranged to receive and hold the de-framed traffic until a completepacket of traffic has been received and to then transmit the de-framedtraffic to an output client signal interface.

A fourth aspect of the invention provides a communications networkcomprising a communications network transmission element as describedabove.

The communications network may be able to reduce the transmission bitrate in response to receiving a client signal at the transmissionelement having a client bit rate which is less than a maximumtransmission bit rate capability of the transmission element. Reducingthe transmission bit rate may reduce power consumption in thetransmission element, particularly in the transmission apparatus. Thepower consumption of the transmission element may therefore be varied tomatch the received traffic load. Traffic transmission may thereforecontinue continuously across the network but at a lower transmission bitrate than the maximum capacity of the network or of transmissionapparatus in the network.

In an embodiment, the communications network further comprises acommunications network element as described above. Backward informationfrom the network element to the transmission element may therefore byprovided to inform the transmission apparatus at the transmissionelement that the network element has lost the bit rate. Causing thetransmission apparatus at the transmission element to transmit trafficat a default transmission bit rate may ease regaining of transmissionbit rate and frame synchronisation at the network element.

A fifth aspect of the invention provides a method of framing traffic fortransmission in a communications network. The method comprises receivinga first client signal having a first client bit rate and comprisingfirst traffic to be transmitted. The first client signal is to betransmitted at a first transmission bit rate. The method furthercomprises receiving a second client signal having a second client bitrate and comprising second traffic to be transmitted. The second clientsignal is to be transmitted at a second transmission bit rate. Themethod further comprises generating and transmitting a control signal.The control signal is arranged to cause a framer to frame a part of thefirst traffic in a payload of a frame. The control signal is furtherarranged to cause the framer to provide information indicative of thesecond transmission bit rate in an overhead of the frame.

Information indicative of the transmission bit rate for a subsequentframe may therefore be sent with a first frame, which may enablereceiver apparatus to prepare to receive traffic at the subsequenttransmission bit rate.

In an embodiment, the method further comprises monitoring an indicatorindicative of said first client bit rate and monitoring an indicatorindicative of said second client bit rate. The method further comprisessetting the first transmission bit rate in dependence on said firstclient bit rate and setting the second transmission bit rate independence on said second client bit rate.

The transmission bit rate may therefore be reduced in response toreceiving a client signal having a client bit rate which is less than amaximum transmission bit rate at which traffic can be transmitted acrossthe network. Reducing the transmission bit rate may reduce powerconsumption during transmission of traffic. The method may enable powerto consumption to be varied to match the received traffic load. Themethod may enable traffic transmission to continue continuously but at alower transmission bit rate than the maximum for the network and maythus enable the method to be used in connection with currently installedwavelength division multiplexed network infrastructure.

In an embodiment, the frame comprises an optical transport networkframe. In an embodiment, the optical transport network frame comprisesan optical transport network frame as defined in ITU-T RecommendationG.709. In an embodiment, the control signal is arranged to cause theframer to provide information indicative of the second transmission bitrate in one of: byte 15 of one of rows 1 and 2 of the overhead of thefirst frame; one of bytes 13 to 15 of row 3 of the overhead of the firstframe; and one of bytes 9 to 14 and 16 of row 4 of the overhead of thefirst frame. The information may therefore be provided in any availablebytes of the overhead which are not being used in connection withtransmission of the traffic, including byte 16 in rows 1 to 4 which areusually allocated for justification control but which are not requiredin this method since justification control is only required whentransporting constant bit rate traffic.

In an embodiment, the control signal is arranged to cause the framer toprovide said information indicative of the second transmission bit ratein a first plurality of said bytes. The first plurality of bytes arelocated in a second plurality of rows of the overhead. This may provideredundancy against transmission errors.

In an embodiment, the information indicative of the second transmissionbit rate comprises a plurality of bits coded to indicate that the secondtransmission bit rate comprises one of: a master bit rate; one half ofthe master bit rate; one third of the master bit rate; and one quarterof the master bit rate. In an embodiment, the information indicative ofthe second transmission bit rate comprises three bits coded as one of:001 to indicate the master bit rate; 010 to indicate one half of themaster bit rate; 100 to indicate one third of the master bit rate; and110 to indicate one quarter of the master bit rate.

In an embodiment, the method comprises receiving each said client signalin an input traffic queue buffer and monitoring the rate at which thequeue buffer is filled. In an embodiment, the rate at which the queuebuffer is filled is monitored by monitoring when a fill level of thequeue buffer reaches a threshold fill level. In an embodiment, the inputtraffic queue buffer comprises a first-in-first-out buffer. Using aninput traffic queue buffer may reduce latency variation because when thequeue buffer is filled traffic is transmitted at a higher transmissionbit rate.

In an embodiment, the indicator comprises a client bit rate signalindicative of the client bit rate. In an embodiment, the client bit ratesignal is received from a network management system.

A sixth aspect of the invention provides a method of controllingtransmission of traffic in a communications network. The methodcomprises receiving a transport signal carrying traffic and having atransmission bit rate. The method further comprises determining whethera value for the transmission bit rate is available. If a value for thetransmission bit rate is not available, the method comprises causing alocal transmitter apparatus to transmit a recovery signal. The recoverysignal is arranged to cause a remote transmission apparatus to transmittraffic at a default transmission bit rate.

Backward information may therefore be sent from the local transmitterapparatus to the remote transmission apparatus to inform the remotetransmission apparatus that a bit rate is not available for thetransport signal. Causing the remote transmission apparatus to transmittraffic at a default transmission bit rate may ease regaining oftransmission bit rate and frame synchronisation.

In an embodiment, the recovery signal is arranged to cause the remotetransmission apparatus to re-transmit the traffic on a transport signalat the default transmission bit rate. The risk of losing traffic forwhich the transmission bit rate was not available may therefore bereduced.

In an embodiment, the method further comprises transmitting the recoverysignal at a pre-determined recovery signal transmission bit rate. Themethod comprises framing the recovery signal in a payload of a frame andproviding a recovery signal identifier in an overhead of the frame. Inan embodiment, the recovery signal identifier comprises two bits codedas 10.

In an embodiment, the communications network is one of comprises anoptical transport network and a synchronous digital hierarchy network.The transmission bit rate is determined by one of an optical channeltransport unit clock rate, an optical channel data unit clock rate andan optical channel payload unit clock rate. In an embodiment, thecommunications network comprises a synchronous digital hierarchynetwork. The transmission bit rate is determined by a synchronoustransport module of the network. A seventh aspect of the inventionprovides a data carrier having computer readable instructions embodiedtherein. The said computer readable instructions are for providingaccess to resources available on a processor. The computer readableinstructions comprise instructions to cause the processor to perform anyof the above steps of the method of transmitting traffic in acommunications network.

An eighth aspect of the invention provides a data carrier havingcomputer readable instructions embodied therein. The said computerreadable instructions are for providing access to resources available ona processor. The computer readable instructions comprise instructions tocause the processor to perform any of the above steps of the method offraming traffic for transmission in a communications network.

A ninth aspect of the invention provides a data carrier having computerreadable instructions embodied therein. The said computer readableinstructions are for providing access to resources available on aprocessor. The computer readable instructions comprise instructions tocause the processor to perform any of the above steps of the method ofcontrolling transmission of traffic in a communications network.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the steps of a method of transmitting traffic in acommunications network according to a first embodiment of the invention;

FIG. 2 shows the steps of a method of transmitting traffic in acommunications network according to a second embodiment of theinvention;

FIG. 3 shows the steps of a method of transmitting traffic in acommunications network according to a third embodiment of the invention;

FIG. 4 shows the steps of a method of transmitting traffic in acommunications network according to a fifth embodiment of the invention;

FIG. 5 shows the steps of a method of transmitting traffic in acommunications network according to a sixth embodiment of the invention;

FIG. 6 is a representation of an optical transport network frame;

FIG. 7 shows the steps of a method of transmitting traffic in acommunications network according to an eighth embodiment of theinvention;

FIG. 8 shows the steps of a method of transmitting traffic in acommunications network according to a tenth embodiment of the invention;

FIG. 9 shows the steps of a method of transmitting traffic in acommunications network according to a twelfth embodiment of theinvention;

FIG. 10 shows the steps of a method of transmitting traffic in acommunications network according to a thirteenth embodiment of theinvention;

FIG. 11 is a schematic representation of a communications networktransmission element according to a fifteenth embodiment of theinvention;

FIG. 12 is a schematic representation of a communications networktransmission element according to a sixteenth embodiment of theinvention;

FIG. 13 is a schematic representation of a communications networktransmission element according to an eighteenth embodiment of theinvention;

FIG. 14 is a schematic representation of a communications networkelement according to a twentieth embodiment of the invention;

FIG. 15 is a schematic representation of a communications networkelement according to a twenty-first embodiment of the invention;

FIG. 16 is a schematic representation of a communications networkelement according to a twenty-second embodiment of the invention;

FIG. 17 is a schematic representation of a communications networkaccording to a twenty-third embodiment of the invention;

FIG. 18 is a schematic representation of a communications networkaccording to a twenty-fourth embodiment of the invention;

FIG. 19 shows the steps of a method of framing traffic for transmissionin a communications network according to a twenty-fifth embodiment ofthe invention;

FIG. 20 shows the steps of a method of framing traffic for transmissionin a communications network according to a twenty-sixth embodiment ofthe invention;

FIG. 21 shows the steps of a method of controlling transmission oftraffic in a communications network according to a twenty-eighthembodiment of the invention; and

FIG. 22 shows the steps of a method of controlling transmission oftraffic in a communications network according to a twenty-ninthembodiment of the invention.

DETAILED DESCRIPTION

FIG. 1 shows the steps of a method 10 of transmitting traffic in acommunications network according to a first embodiment of the invention.

The method 10 comprises receiving a client signal having a client bitrate and comprising traffic to be transmitted 12 and monitoring anindicator indicative of the client bit rate 14. The method 10 furthercomprises setting a transmission bit rate at which to transmit thetraffic 16. The transmission bit rate is set in dependence on the clientbit rate. A control signal arranged to cause a transmission apparatus totransmit the traffic at the transmission bit rate is generated andtransmitted 18.

The method may be applied to any type of communications network,including an optical transport network (OTN) and a synchronous digitalhierarchy (SDH) network.

The client signal may comprise one signal carrying traffic to betransmitted or it may comprise an aggregate of a plurality of signals,each carrying traffic to be transmitted. The client bit rate maytherefore comprise the bit rate of a single signal or it may comprisethe gross bit rate of a plurality of aggregated signals.

The transmission bit rate refers to the rate at which the transmissionapparatus transmits a single stream of traffic. The control signaltherefore controls the actual rate at which the transmission apparatustransmits traffic and not an effective transmission bit rate such as canbe achieved in time division multiplexed (TDM) transmission in whichmultiple streams of traffic are transmitted in different time slots. Aneffective transmission bit rate for a traffic stream can be achieved inTDM transmission which is different to the rate at which traffic istransmitted within each time slot, i.e. the rate at which thetransmission apparatus transmits data, the effective transmission bitrate being dependent upon the number and allocation of time slots inwhich a stream of traffic is transmitted.

The method may enable the transmission bit rate to be reduced inresponse to receiving a client signal having a client bit rate which isless than a maximum transmission bit rate at which traffic can betransmitted across the network. Reducing the transmission bit rate atwhich the transmission apparatus transmits the traffic may reduce thepower consumption of the transmission apparatus during transmission oftraffic. The power consumption of the transmission apparatus maytherefore be varied to match the received traffic load. The method mayenable traffic transmission to continue continuously but at a lowertransmission bit rate than the maximum for the network or the capabilityof the transmission apparatus and may thus enable the method to be usedin connection with currently installed wavelength division multiplexednetwork infrastructure.

The steps of a method 20 of transmitting traffic in a communicationsnetwork according to a second embodiment of the invention are shown inFIG. 2. The method 20 is similar to the method 10 of FIG. 1, with thefollowing modifications. The same reference numbers are retained forcorresponding steps.

In this embodiment, the transmission bit rate is set by determining adivide ratio for a master bit rate in dependence on the client bit rate22. The control signal comprises a clock signal generated by receiving amaster clock signal at the master bit rate and dividing the master clocksignal by the divide ratio 24. The control signal is then transmitted toa transmission apparatus to cause the transmission apparatus to transmitthe traffic at the transmission bit rate which has been set 26.

This may provide a simple mechanism for setting the transmission bitrate and generating the control signal. The transmission bit rate isvaried by varying the divide ratio while the master clock signal has aconstant master bit rate. This may ease clock recovery at a receiverwhich can run at a constant bit rate locked to an incoming signal.

The steps of a method 30 of transmitting traffic in a communicationsnetwork according to a third embodiment of the invention are shown inFIG. 3. The method 30 is similar to the method 20 of FIG. 2, with thefollowing modifications. The same reference numbers are retained forcorresponding steps.

In this embodiment, the transmission bit rate is set by selecting adivide ratio from a set of divide ratios in dependence on the client bitrate 32. This may further simplify operation of the method as it is onlynecessary to select an appropriate one of a set of divide ratios ratherthan determining a divide ratio.

In a fourth embodiment, the step of selecting a divide ratio furthercomprises selecting a plurality of divide ratios from the set of divideratios. An intermediate transmission bit rate may then set byperiodically switching between each of the selected divide ratios. Theintermediate transmission bit rate is an average of the transmission bitrates at the selected divide ratios.

The steps of a method 40 of transmitting traffic in a communicationsnetwork according to a fifth embodiment of the invention are shown inFIG. 4.

The method 40 comprises receiving a first client signal having a firstclient bit rate and comprising first traffic to be transmitted 42 andsubsequently receiving a second client signal having a second client bitrate and comprising second traffic to be transmitted 44. A firsttransmission bit rate is set in dependence on the first client bit rate46 and a second transmission bit rate is set in dependence on the secondclient bit rate 48. A first control signal is generated and transmittedwhich is arranged to cause a framer to frame the first traffic into oneor more first frames at the first transmission bit rate 50. A secondcontrol signal is generated and transmitted which is arranged to causethe framer to frame the second traffic into one or more subsequentframes at the second transmission bit rate 52. Where the secondtransmission bit rate is different to the first transmission bit rate,the second control signal is further arranged to cause the framer toimplement the change from the first transmission bit rate to the secondtransmission bit rate at a frame boundary. Changing the transmission bitrate only at a frame boundary may ensure that a clean change over isachieved from the first transmission bit rate to the second transmissionbit rate and may prevent spillage of traffic from a first frame into asubsequent frame.

FIG. 5 shows the steps of a method 60 of transmitting traffic in acommunications network according to a sixth embodiment of the invention.The method 40 of this embodiment is similar to the method 40 of theprevious embodiment, with the following modifications. The samereference numbers are retained for corresponding steps.

In this embodiment, the first control signal is arranged to cause theframer to frame the first traffic in a payload of one or more firstframes 62. The first control signal is additionally arranged to causethe framer to provide information indicative of the second transmissionbit rate in an overhead of a first frame 62. This may enable a receiverreceiving the traffic to track changes in the transmission bit rate andto prepare to receive traffic at a different transmission bit rate.

In a seventh embodiment of the invention the frames into which thetraffic is framed are optical transport network (OTN) frames. Aschematic representation of an OTN frame which meets the requirements ofITU-T Recommendation G.709 is shown in FIG. 6.

The OTN frame 70 comprises four rows 72 each of which comprise 4080bytes (columns) 74. Bytes 1 to 14 of each row comprise the opticalchannel transport unit (OTU) overhead and the optical channel data unit(ODU) overhead 76. Bytes 15 and 16 of each row comprise the opticalchannel payload unit (OPU) overhead 78. The payload 80 of the frame 70comprises bytes 17 to 3824 of each row and forward error correction(FEC) information 82 is allocated to bytes 3825 to 4080 of each row.

In this example, the information indicative of the second transmissionbit rate may be provided in any available bytes of the overhead whichare not being used in connection with transmission of the traffic. Inparticular, this information may be provided in one of: byte 15 of oneof rows 1 and 2; one of bytes 13 to 15 of row 3; and one of bytes 9 to14 and 16 of row 4.

Redundancy against transmission errors may be provided by using bytes intwo or more of the rows 72 of the overhead 76, 78.

The information indicative of the second transmission bit rate comprisesa plurality of bits coded to indicate that the second bit rate comprisesone of: a master bit rate; one half of the master bit rate; one third ofthe master bit rate; and one quarter of the master bit rate.

In this example, 001 is used to indicate the master bit rate; 010 toindicate one half of the master bit rate; 100 to indicate one third ofthe master bit rate; and 110 to indicate one quarter of the master bitrate.

FIG. 7 shows the steps of a method 90 of transmitting traffic in acommunications network according to an eighth embodiment of theinvention. The method 90 of this embodiment is similar to the method 10of FIG. 1, with the following modifications. The same reference numbersare retained for corresponding steps.

In this embodiment, the method 90 further comprises receivingtransmitted traffic at receiver apparatus 92 and determining whether avalue for the transmission bit rate is available 94. If a value for thetransmission bit rate is not available, the method further comprisescausing a further transmission apparatus to generate and transmit arecovery signal 96. The recovery signal is arranged to cause thetransmission apparatus to transmit traffic at a default transmission bitrate. Backward information may therefore be sent from the receiverapparatus to the transmission apparatus to inform the transmissionapparatus that the receiver apparatus has lost the bit rate.Transmitting traffic at a default transmission bit rate may easeregaining of transmission bit rate and frame synchronisation at thereceiver apparatus.

A ninth embodiment of the invention provides a method of transmittingtraffic in a communications network having the same method steps as theprevious embodiment, with the following modification. In thisembodiment, the recovery signal is arranged to cause the remotetransmission apparatus to re-transmit the traffic on a transport signalat the default transmission bit rate. The traffic for which a bit ratewas not available is therefore re-transmitted and the risk of losing thetraffic may therefore be reduced.

The steps of a method 100 of transmitting traffic in a communicationsnetwork according to a tenth embodiment of the invention are shown inFIG. 8. The method 100 of this embodiment is similar to the method 90 ofFIG. 7, with the following modifications. The same reference numbers areretained for corresponding steps.

In this embodiment, the further transmission apparatus is arranged togenerate a recovery signal arranged to cause the transmission apparatusto transmit traffic at a default transmission bit rate 102. The recoverysignal is framed in a payload of a frame and a recovery signalidentifier is provided in an overhead of the frame 104. The recoverysignal is transmitted at a pre-determined recovery signal transmissionbit rate 104. An eleventh embodiment of the invention provides a methodof transmitting traffic in a communications network which is similar tothe method 100 of the previous embodiment. The method of this embodimentcomprises the same steps as the method 100, with the recovery signalidentifier comprising two bits coded as 10 which are provided in theoverhead of the frame 104.

FIG. 9 shows the steps of a method 110 of transmitting traffic in acommunications network according to a twelfth embodiment of theinvention. The method 110 of this embodiment is similar to the method 10of FIG. 1, with the following modifications. The same reference numbersare retained for corresponding steps.

The indicator indicative of the client bit rate comprises a client bitrate signal. The method of this embodiment comprises receiving a clientbit rate signal indicative of the client bit rate from a networkmanagement system.

FIG. 10 shows the steps of a method 300 of transmitting traffic in acommunications network according to a thirteenth embodiment of theinvention. The method 300 of this embodiment is similar to the method 10of FIG. 1, with the following modifications. The same reference numbersare retained for corresponding steps.

In this embodiment, the client bit rate is determined by receiving theclient signal in an input traffic queue buffer 302 and monitoring therate at which the buffer is filled 304.

The transmission bit rate is set in dependence on the fill rate 306.

Using an input traffic queue buffer may reduce latency variation becausewhen the queue is filled traffic is transmitted at a higher transmissionbit rate.

A fourteenth embodiment of the invention provides a method oftransmitting traffic in a communications network which is similar to themethod 300 of FIG. 10.

In this embodiment, the rate at which the queue buffer is filled ismonitored by monitoring when the fill level of the queue buffer reachesa threshold value.

A fifteenth embodiment of the invention provides a method oftransmitting traffic in a communications network which is similar to themethod 20 of FIG. 2. The method of this embodiment comprises the samesteps as the method 20, with the step of determining a divide ratiofurther comprising obtaining a temperature of the transmission apparatusand the divide ratio is set in dependence on the temperature. Reducingthe transmission bit rate in dependence on the ambient temperature mayreduce the power required for cooling the transmission apparatus. Themethod may therefore be used to throttle the transmission bit rate ifthe ambient temperature rises above a threshold value.

Referring to FIG. 11, a sixteenth embodiment of the invention provides acommunications network transmission element 120. The communicationsnetwork transmission element 120 comprises an input 122, client signalmonitoring apparatus 126, transmission bit rate control apparatus 128and transmission apparatus 130.

The input 122 is arranged to receive a client signal 124 having a clientbit rate and comprising traffic to be transmitted. The client signalmonitoring apparatus 126 is arranged to monitor an indicator indicativeof the client bit rate and to transmit a client bit rate signal 134 tothe transmission bit rate control apparatus. The transmission bit ratecontrol apparatus 128 is arranged to set a transmission bit rate atwhich to transmit the received traffic. The transmission bit rate is setin dependence on the client bit rate as indicated by the client bit ratesignal 134. The transmission bit rate control apparatus is furtherarranged to generate and transmit a control signal 136 arranged to causethe transmission apparatus 130 to transmit the traffic at thetransmission bit rate. The transmission apparatus 130 is arranged toreceive the control signal 136 and to generate and transmit a transportsignal 132 carrying the traffic.

The transmission element 120 may be able to reduce the transmission bitrate in response to receiving a client signal having a client bit ratewhich is less than a maximum transmission bit rate capability of thetransmission element. In this way the power consumption of thetransmission apparatus may be reduced and may be varied to Match thereceived traffic load. The transmission element 120 is able to transmittraffic continuously but at a lower transmission bit rate than itsmaximum capability. The transmission element 120 may therefore be usedwithin currently installed WDM network infrastructure.

A seventeenth embodiment of the invention provides a communicationsnetwork transmission element 140, as shown in FIG. 12. The transmissionelement 140 is similar to the transmission element 120 of FIG. 10, withthe following modifications. The same reference numbers are retained forcorresponding features.

In this embodiment, the transmission element 140 comprises a sourcetermination of an optical transport network (OTN), as defined by ITU-TRecommendation G.709.

The source termination 140 comprises a client signal interface 142arranged to receive a client signal having a client bit rate andcomprising traffic for transmission. The client signal interrogationapparatus 126 comprises a queue buffer, which in this example takes theform of a first-in-first-out (FIFO) buffer which is arranged to receivethe client signal from the client signal interface 142. As the clientsignal is received at the FIFO buffer 126 the fill level of the bufferis monitored and when the fill level reaches a threshold value a filllevel signal 127 that the threshold value has been reached istransmitted to the transmission bit rate control apparatus 128.

In this example, the transmission bit rate control apparatus 128comprises a controller 144, a clock 146 and a divider 148. Thecontroller 144 is arranged to receive the fill level signal 127 and todetermine a divide ratio in dependence on the fill level. The clock 146is arranged to generate a master clock signal 147 having a master bitrate. The divider 148 is arranged to receive a divide ratio signal 145from the controller 144 and to divide the master clock signal 147 at thedivide ratio to form a control signal 156.

The transmission apparatus 130 comprises an adaption layer 150, forexample generic framing procedure (GFP), a framer 152 and a modulator154. The adaption layer 150 is arranged to receive traffic fortransmission from the FIFO buffer 126 and to prepare the traffic forframing by the framer 152, including removing jitter and adding data tobe provided in the overhead of the frame.

The transmission bit rate is varied by varying the divide ratio, independence on the fill level, whilst the master clock signal 147 has aconstant master bit rate.

In this example, the divide ratio is selected from a set of divideratios provided within the controller 144, the divide ratio beingselected from the set in dependence on the fill level.

An eighteenth embodiment of the invention provides a communicationsnetwork transmission element 160, as shown in FIG. 13. The transmissionelement 160 of this embodiment again comprises a source termination ofan OTN and is similar to the source termination 140 of FIG. 12, with thefollowing modifications. The same reference numbers are retained forcorresponding features.

In this embodiment, the source termination 160 is arranged to receive afirst client signal 166 having a first client bit rate and comprisingfirst traffic to be transmitted and to subsequently receive a secondclient signal 167 having a second client bit rate and comprising secondto be transmitted. The controller 144 is arranged to set a firsttransmission bit rate in dependence on the first client bit rate atwhich to transmit the first traffic. This is done by the controller 144determining a divide ratio in dependence on the fill level signal 127received from the FIFO buffer 126 for the first client signal. Thecontroller 144 is similarly arranged to determine a divide ratio for thesecond client signal 167, to set a second transmission bit rate at whichto transmit the second traffic. The clock signal 147 is divided by thefirst divide ratio to provide a first control signal 168 arranged tocause the framer 152 to frame the first traffic into one or more firstframes at the first transmission bit rate. The clock signal 147 issubsequently divided by the second divide ratio to generate a secondcontrol signal 169 arranged to cause the framer to frame the secondtraffic into one or more subsequent frames at the second transmissionbit rate.

Where the second transmission bit rate is different to the firsttransmission bit rate, the second control signal 169 is further arrangedto cause the framer 152 to implement a change from the firsttransmission bit rate to the second transmission bit rate at a frameboundary. Causing the bit rate to change only at a frame boundary mayensure that a clean change over is achieved from the first transmissionbit rate to the second transmission bit rate and may prevent spillage oftraffic from a first frame into a subsequent frame.

The controller 144 may be further arranged to generate and transmit abit rate code signal 151 indicative of the first transmission bit rateand the second transmission bit rate, which is transmitted to the framer152.

The first control signal 168 is arranged to cause the framer 152 toframe the first traffic in a payload of one or more first frames. Thefirst control signal is further arranged to cause the framer 152 toprovide information indicative of the second transmission bit rate in anoverhead of at least one first frame.

The source termination 160 further comprises a receiver 162 arranged toreceive a recovery signal 164. The recovery signal is arranged to causethe transmission apparatus 130 to transmit a transport signal carryingtraffic at a default transmission bit rate. The recovery signal causesthe framer 152 to frame the traffic at the default transmission bitrate.

The source termination 160 is therefore able to receive backwardinformation from a remote transmission apparatus to which the trafficcarrying signal 152 has been transmitted, to inform the sourcetermination 160 that the remote transmission apparatus has lost the bitrate.

In a nineteenth embodiment, the source termination 160 may furthercomprise a thermometer arranged to measure a temperature of the sourcetermination 160, which may be the ambient temperature. The controller144 is arranged to receive a temperature signal from the thermometerindicative of the ambient temperature and the controller 144 is furtherarranged to set the divide ratio in dependence on the temperature. Thetransmission bit rate can therefore be reduced in dependence on theambient temperature where the temperature exceeds a threshold value, andthis is desirable to reduce the amount of cooling required for thesource termination. This may be done when the client bit rate is lowerthan the maximum transmission bit rate capability of the transmissionapparatus.

A twentieth embodiment of the invention provides a communicationsnetwork element 170, as shown in FIG. 14.

The communications network element 170 comprises an input 172, transportsignal interrogation apparatus 176 and local transmission apparatus 178.The input 172 is arranged to receive a transport signal 174 carryingtraffic having a transmission bit rate. The transport signalinterrogation apparatus 176 is arranged to determine whether a value forthe transmission bit rate is available. If a value is not available forthe transmission bit rate, the transport signal interrogation apparatusis further arranged to generate a recovery signal 180 arranged to causea remote transmission apparatus to transmit a transport signal carryingtraffic at a default transmission bit rate. The local transmissionapparatus 178 is arranged to transmit the recovery signal 180.

The communications network element 170 may therefore provide backwardinformation to a remote transmission apparatus, for example at a furthernetwork element, to inform the transmission apparatus that the networkelement has lost the bit rate. Causing the remote transmission apparatusto transmit, or re-transmit, the traffic at a default transmission bitrate may ease regaining of transmission bit rate and framesynchronization at the network element 170.

FIG. 15 shows a communications network element 190 according to atwenty-first embodiment of the invention. The communications networkelement 190 is similar to the network element 170 of FIG. 14, with thefollowing modifications. The same reference numbers are retained forcorresponding features.

The communications network element 190 of this embodiment comprises asink termination of an OTN, as defined by ITU-T Recommendation G.709.

The sink termination 190 comprises a receiver soft decision equalizer192 arranged to receive the transport signal 174. The receiver 192 isarranged to convert the received optical signal into an electricalsignal to transmit that to a deframer 194. The deframer 194 is arrangedto deframe the traffic and to transmit it to an adaption layer 196arranged to remove the overhead and to transmit the traffic to a queuebuffer 198. The queue buffer 198 is arranged to receive and hold thedeframed traffic until a complete packet of traffic has been received,following which the traffic is transmitted to an output client signalinterface arranged to convert the traffic into an output client signalfor onwards transmission or processing.

The deframer 194 is arranged to retrieve information from the frameoverhead relating to the transmission bit rate for a subsequent frameand to transmit a bit rate code signal 206 to the transport signalinterrogation apparatus 176, which is arranged to store the transmissionbit rate for future reference.

On receipt of a transport signal carrying traffic 124, the transportsignal interrogation apparatus 176 is arranged to determine whether avalue for the transmission bit rate for the signal is available. If thetransmission bit rate has not been received previously or has been lost,the transport signal interrogation apparatus 176 is arranged to generatea recovery signal 180, as described above.

In this example, the transport signal interrogation apparatus 176 isfurther arranged to determine a divide ratio for generating a controlsignal 208 for the deframer 194. The divide ratio is set according tothe bit rate and a divide ratio control signal 209 is transmitted to adivider 204. The control signal 208 for the deframer is generated bydividing a clock signal generated by a master clock 202 by the divideratio.

A communications network element 210 according to a twenty-secondembodiment of the invention is shown in FIG. 16. The network element 210of this embodiment is similar to the network element 170 of FIG. 13,with the following modifications. The same reference numbers areretained for corresponding features.

In this embodiment, the local transmission apparatus 178 is arrangedtransmit the recovery signal 212 at a pre-determined recovery signaltransmission bit rate. The local transmission apparatus 178 is arrangedto frame the recovery signal in a payload of a frame and to provide arecovery signal identifier in an overhead of the frame. The recoverysignal identifier comprises two bits coded as 10.

A twenty-third embodiment of the invention provides a communicationsnetwork 220 as shown in FIG. 17.

The communications network 220 comprises a communications networktransmission element 120 as shown in FIG. 11, a transport link 222 and acommunications network element 170 as shown in FIG. 14.

FIG. 18 shows a communications network 230 according to a twenty-fourthembodiment of the invention.

The communications network 230 comprises a source termination 140 asshown in FIG. 12, a transport link 232 and a sink termination 190 asshown in FIG. 15.

A twenty-fifth embodiment of the invention provides a method 240 offraming traffic for transmission in a communications network. The stepsof the method 240 are shown in FIG. 19.

The method 240 comprises receiving a first client signal having a firstclient bit rate and comprising first traffic to be transmitted. at afirst transmission bit rate 242 and receiving a second client signalhaving a second client bit rate and comprising second traffic to betransmitted at a second transmission bit rate 244. The method furthercomprises generating and transmitting a control signal 246. The controlsignal is arranged to cause a framer to frame part of the first trafficin a payload of a frame. The control signal is further arranged to causethe framer to provide information indicative of the second transmissionbit rate in an overhead of the frame.

Information indicative of the transmission bit rate for a subsequentframe may therefore be sent with a first frame, which may enablereceiver apparatus to prepare to receive traffic at the subsequenttransmission bit rate.

The steps of a method 250 of framing traffic for transmission in acommunications network according to a twenty-sixth embodiment of theinvention are shown in FIG. 20.

The method 250 comprises receiving a first client signal having a firstclient bit rate and comprising first traffic to be transmitted 252 andreceiving a second client signal having a second client bit rate andcomprising second traffic to be transmitted 254. The method comprisessetting a first transmission bit rate at which to transmit the firsttraffic in dependence on the first client bit rate 256 and setting asecond transmission bit rate at which to transmit the second traffic independence on the second client bit rate 258.

The method 250 further comprises generating and transmitting a controlsignal arranged to cause a framer to frame a part of the first trafficin a payload of a frame 260. The control signal is further arranged tocause the framer to provide information indicative of the secondtransmission bit rate in an overhead of the frame 260.

The transmission bit rate may therefore be reduced in response toreceiving client signal having a client bit rate which is less than amaximum transmission bit rate at which traffic can be transmitted acrossthe network. Reducing the transmission bit rate may reduce powerconsumption during transmission of traffic. The method may enable powerconsumption to be varied to match the received traffic load. The methodmay enable traffic transmission to continue continuously but at a lowertransmission bit rate than the maximum for the network and may thusenable the method to be used in connection with currently installedwavelength division multiplexed network infrastructure.

In a twenty-seventh embodiment of the invention the frames into whichthe traffic is framed are optical transport network (OTN) frames, asdescribed above in relation to FIG. 6.

The information indicative of the second transmission bit rate may beprovided in any available bytes of the overhead which are not being usedin connection with transmission of the traffic. In particular, thisinformation may be provided in one of: byte 15 of one of rows 1 and 2;one of bytes 13 to 15 of row 3; and one of bytes 9 to 14 and 16 of row4.

Redundancy against transmission errors may be provided by using bytes intwo or more of the rows 72 of the overhead 76, 78.

The information indicative of the second transmission bit rate comprisesa plurality of bits coded to indicate that the second bit rate comprisesone of: a master bit rate; one half of the master bit rate; one third ofthe master bit rate; and one quarter of the master bit rate.

In this example, 001 is used to indicate the master bit rate; 010 toindicate one half of the master bit rate; 100 to indicate one third ofthe master bit rate; and 110 to indicate one quarter of the master bitrate.

The steps of a method 270 of controlling transmission of traffic in acommunications network according to a twenty-eighth embodiment of theinvention are shown in FIG. 21.

The method 270 comprises receiving a transport signal carrying trafficand having a transmission bit rate 272. The method further comprisesdetermining whether a value for the transmission bit rate is available.If a value for the transmission bit rate is not available, the methodcomprises causing a local transmitter apparatus to transmit a recoverysignal 276. The recovery signal is arranged to cause a remotetransmission apparatus to transmit traffic at a default transmission bitrate.

Backward information may therefore be sent from the local transmitterapparatus to the remote transmission apparatus to inform the remotetransmission apparatus that a bit rate is not available for thetransport signal. Causing the remote transmission apparatus to transmittraffic at a default transmission bit rate may ease regaining oftransmission bit rate and frame synchronisation.

The steps of a method 280 of controlling transmission of traffic in acommunications network according to a twenty-ninth embodiment of theinvention are shown in FIG. 22.

The method 280 of this embodiment is similar to the method 270 of theprevious embodiment with the following modifications. The same referencenumbers are retained for corresponding steps.

In this embodiment, the method comprises causing the local transmitterapparatus to generate a recovery signal arranged to cause the remotetransmission apparatus to transmit traffic at a default transmission bitrate 282. The recovery signal is framed in a payload of a frame and arecovery signal identifier is provided in an overhead of the frame 284.The recovery signal is transmitted at a pre-determined recovery signaltransmission bit rate 284.

A thirtieth embodiment of the invention provides a data carrier havingcomputer readable instructions embodied therein. The said computerreadable instructions are for providing access to resources available ona processor. The computer readable instructions comprise instructions tocause the processor to perform the steps of any of the methods oftransmitting traffic in a communications network according of the firstto thirteenth embodiments described above.

The data carrier may comprise a memory device, such a compact disc,digital versatile disc or electronic memory, or may comprise acommunications signal.

A thirty-first embodiment of the invention provides a data carrierhaving computer readable instructions embodied therein. The saidcomputer readable instructions are for providing access to resourcesavailable on a processor. The computer readable instructions compriseinstructions to cause the processor to perform the steps of any of themethods of framing traffic for transmission in a communications networkof the twenty-third to twenty-fifth embodiments described above.

The data carrier may comprise a memory device, such a compact disc,digital versatile disc or electronic memory, or may comprise acommunications signal.

A thirty-second embodiment of the invention provides a data carrierhaving computer readable instructions embodied therein. The saidcomputer readable instructions are for providing access to resourcesavailable on a processor. The computer readable instructions compriseinstructions to cause the processor to perform the steps of any of themethod of controlling transmission of traffic in a communicationsnetwork of the twenty-sixth to twenty-seventh embodiments describedabove.

The data carrier may comprise a memory device, such a compact disc,digital versatile disc or electronic memory, or may comprise acommunications signal.

The invention claimed is:
 1. A method of transmitting traffic in acommunications network, the method comprising: receiving a client signalin a buffer, the client signal having a client bit rate and comprisingtraffic to be transmitted by a transmission element; monitoring anindicator indicative of said client bit rate, wherein the indicatorindicative of said client bit rate corresponds to a fill rate or filllevel of the buffer; setting a transmission bit rate of the transmissionelement at which to transmit the traffic, the transmission bit ratebeing set in dependence on said client bit rate determined frommonitoring the indicator; wherein the transmission bit rate of thetransmission element is reduced in response to receiving a said clientbit rate which is less than a maximum transmission bit rate capabilityof the transmission element; and varying a clock rate to cause atransmission apparatus to transmit the traffic at the transmission bitrate.
 2. The method as claimed in claim 1, wherein the transmission bitrate set by varying the clock rate comprises determining a divide ratiofor a master bit rate in dependence on said client bit rate andgenerating and transmitting a control signal that comprises a clocksignal generated by receiving a master clock signal at the master bitrate and dividing the master clock signal by the divide ratio.
 3. Themethod as claimed in claim 1, wherein the method comprises: receiving afirst client signal having a first client bit rate and comprising firsttraffic to be transmitted and subsequently receiving a second clientsignal having a second client bit rate and comprising second traffic tobe transmitted; setting a first transmission bit rate in dependence onthe first client bit rate and setting a second transmission bit rate independence on the second client bit rate; generating and transmitting afirst control signal arranged to cause a framer to frame the firsttraffic into one or more first frames at the first transmission bit rateand a second control signal arranged to cause the framer to frame thesecond traffic into one or more subsequent frames at the secondtransmission bit rate, and, where the second transmission bit rate isdifferent to the first transmission bit rate, the second control signalis further arranged to cause the framer to implement the change from thefirst transmission bit rate to the second transmission bit rate at aframe boundary.
 4. The method as claimed in claim 3, wherein the firstcontrol signal is arranged to cause the framer to frame a part of thefirst traffic in a payload of said first frame and to cause the framerto provide information indicative of the second transmission bit rate inan overhead of said first frame.
 5. The method as claimed in claim 4,wherein said first frame comprises an optical transport network frame.6. The method as claimed in claim 1, wherein the method furthercomprises receiving transmitted traffic at receiver apparatus anddetermining whether a value for the transmission bit rate is available,and, if a value for the transmission bit rate is not available, causinga further transmission apparatus to generate and transmit a recoverysignal arranged to cause the transmission apparatus to transmit trafficat a default transmission bit rate.
 7. The method as claimed in claim 6,wherein the recovery signal is arranged to cause a remote transmissionapparatus to re-transmit the traffic on a transport signal at thedefault transmission bit rate.
 8. The method as claimed in claim 6,wherein the recovery signal is transmitted at a pre-determined recoverysignal transmission bit rate and the method comprises framing therecovery signal in a payload of a frame and providing a recovery signalidentifier in an overhead of the frame.
 9. The method as claimed inclaim 1, wherein the indicator comprises a client bit rate signal.
 10. Acommunications network transmission element comprising: an inputarranged to receive a client signal having a client bit rate andcomprising traffic to be transmitted by a transmission element; clientsignal monitoring apparatus arranged to monitor an indicator indicativeof said client bit rate, wherein the client signal monitoring apparatuscomprises a buffer arranged to receive the client signal and theindicator indicative of said client bit rate corresponds to a fill rateor fill level of the buffer; transmission bit rate control apparatusarranged to set a transmission bit rate of the transmission element atwhich to transmit the received traffic, the transmission bit rate beingset in dependence on said client bit rate determined from monitoring theindicator, wherein: the transmission bit rate of the transmissionelement is reduced in response to receiving a said client bit rate whichis less than a maximum transmission bit rate capability of thetransmission element, and the transmission bit rate control apparatusbeing further arranged to vary a clock rate arranged to cause atransmission apparatus to transmit the traffic at the transmission bitrate; and the transmission apparatus arranged to receive the clock rateand to generate and transmit a transport signal carrying the traffic.11. The communications network transmission element as claimed in claim10, wherein to vary the clock rate, the transmission bit rate controlapparatus comprises a controller arranged to determine a divide ratio independence on said client bit rate, a clock arranged to generate amaster clock signal having a master bit rate, and a divider arranged toreceive the divide ratio and to divide the master clock signal at thedivide ratio to form a control signal.
 12. The communications networktransmission element as claimed in claim 10 wherein the input isarranged to receive a first client signal having a first client bit rateand comprising first traffic to be transmitted and to subsequentlyreceive a second client signal having a second client bit rate andcomprising second traffic to be transmitted, and the transmissionapparatus comprises a framer, and wherein the transmission bit ratecontrol apparatus is arranged to set a first transmission bit rate independence on the first client bit rate at which to transmit the firsttraffic and to set a second transmission bit rate in dependence on thesecond client bit rate at which to transmit the second traffic, and thetransmission bit rate control apparatus is further arranged to generateand transmit a first control signal arranged to cause the framer toframe the first traffic into one or more first frames at the firsttransmission bit rate and a second control signal arranged to cause theframer to frame the second traffic into one or more subsequent frames atthe second transmission bit rate, and, where the second transmission bitrate is different to the first transmission bit rate, the second controlsignal is further arranged to cause the framer to implement the changefrom the first transmission bit rate to the second transmission bit rateat a frame boundary.
 13. The communications network transmission elementas claimed in claim 12, wherein the first control signal is arranged tocause the framer to frame a part of the first traffic in a payload of asaid first frame and to cause the framer to provide informationindicative of the second transmission bit rate in an overhead of thefirst frame.
 14. The communications network transmission element asclaimed in claim 12, wherein said first frame comprises an opticaltransport network frame.
 15. The communications network transmissionelement as claimed in claim 10, wherein the client signal monitoringapparatus is arranged to monitor a client bit rate signal indicative ofthe client bit rate.
 16. The communications network transmission elementas claimed claim 10, wherein the communications network transmissionelement further comprises a receiver arranged to receive a recoverysignal arranged to cause the transmission apparatus to transmit atransport signal carrying traffic at a default bit rate.
 17. Acommunications network comprising: a communication network transmissionelement including: an input arranged to receive a client signal in abuffer, the client signal having a client bit rate and comprisingtraffic to be transmitted by a transmission element; client signalmonitoring apparatus arranged to monitor an indicator indicative of saidclient bit rate, wherein the indicator indicative of said client bitrate corresponds to a fill rate or fill level of the buffer;transmission bit rate control apparatus arranged to: set a transmissionbit rate of the transmission element at which to transmit the receivedtraffic, the transmission bit rate being set in dependence on saidclient bit rate determined from monitoring the indicator, wherein thetransmission bit rate of the transmission element is reduced in responseto receiving a said client bit rate which is less than a maximumtransmission bit rate capability of the transmission element; vary aclock rate arranged to cause a transmission apparatus to transmit thetraffic at the transmission bit rate; and receive the clock rate andtransmit a transport signal carrying the traffic; and a communicationnetwork element including: an input arranged to receive a transportsignal carrying traffic and having a transmission bit rate: transportsignal interrogation apparatus arranged to determine whether a value forthe transmission bit rate is available and being further arranged to, ifa value for the transmission bit rate is not available, generate arecovery signal arranged to cause a remote transmission apparatus totransmit a transport signal carrying traffic at a default transmissionbit rate; and local transmission apparatus arranged to transmit therecovery signal.
 18. A method of framing traffic for transmission in acommunications network, the method comprising: receiving a first clientsignal having a first client bit rate and comprising first traffic to betransmitted at a first transmission bit rate by a transmission element;receiving a second client signal in a buffer, the second client signalhaving a second client bit rate and comprising second traffic to betransmitted at a second transmission bit rate by the transmissionelement, the second transmission bit rate being set in dependence onsaid second client bit rate determined from monitoring a fill rate orfill level of the buffer, wherein the second transmission bit rate ofthe transmission element is reduced in response to the second client bitrate being less than a maximum transmission bit rate capability of thetransmission element; generating and transmitting a control signalarranged to cause a framer to frame a part of the first traffic in apayload of a frame and to cause the framer to provide informationindicative of the second transmission bit rate in an overhead of theframe; varying a clock rate to cause the first traffic to transmit atthe first transmission bit rate and the second traffic to transmit atthe second transmission bit rate.
 19. The method as claimed in claim 18,wherein the frame comprises an optical transport network frame.